The invention relates to a tank venting system for a fuel tank of a vehicle, having a vent line for discharge of gaseous hydrocarbons from the fuel tank during refueling thereof. Gaseous hydrocarbons originating from the fuel tank can be introduced in a filler pipe of the fuel tank via a recirculation line. The tank venting system includes in addition a valve for changing a pass-through cross section of the recirculation line, and a filter for absorbing gaseous hydrocarbons via the vent line. The invention further relates to a method of venting a fuel tank of a vehicle during refueling thereof.
U.S. Pat. No. 4,762,156 A describes a tank venting system with a vent line to connect a fuel tank with an activated charcoal filter. When the fuel tank is filled with fuel via a filler pipe, gaseous hydrocarbons are displaced from the fuel tank and are routed via the vent line into the activated charcoal filter where they are collected. A fraction of the fuel vapors are however returned via a recirculation line back into the filler pipe to reduce the exposure of the activated charcoal filter to fuel vapors. Arranged in the recirculation line is a valve in which a spring imposes forces on a diaphragm so as to keep the valve closed. The spring is accommodated in a space of the valve which space is in communication with the filler pipe via the recirculation line. As a result of a negative pressure in the filler pipe, caused by incoming fuel into the filler pipe, also the space in the valve is subject to a pressure below the atmospheric pressure and the valve opens. An opening width of the valve increases hereby with increase in the negative pressure in the filler pipe. This negative pressure is especially high when a particularly large amount of fuel flows through the filler pipe. When the negative pressure in the filler pipe is low, i.e. when the volume flow of fuel through the filler pipe is small, the valve opens only when the negative pressure is able to overcome the force of the spring. In this way, escape of fuel vapors to the outside via the filler pipe should be prevented when the volume flow is small.
Such a tank venting system is disadvantageous because an unwanted escape of gaseous hydrocarbons from the tank venting system can still be encountered.